This invention relates to novel poly(alkenylpentaboranes) useful as ceramic precursors, to methods of preparing such poly(alkenylpentaboranes), to methods of preparing ceramics such as boron carbide and boron nitride from such poly(alkenylpentaboranes), and to various objects made from such ceramics.
The production of various carbon products, such as carbon fibers, by pyrolysis of carbon-based polymers is widely practiced. The extension of this technology to prepare ceramics by pyrolysis of organometallic polymers or other precursors is now attracting considerable attention. This interest stems from the importance of ceramic products such as SiC, Si.sub.3 N.sub.4, B.sub.4 C and BN and from the potentially versatile and practical processing of these important ceramics that polymer pyrolysis allows. It is being recognized that a variety of polymers can be made consisting of one or more of several toms such as Si, B and N in addition to, or in lieu of, carbon atoms, and that, using these new polymers, it should be possible to make the ceramic products mentioned above in direct analogy with the polymer pyrolysis for processing of carbon materials. Rice, R. W. Ceram. Bull., 62, 889-892 (1983); Wynne, K. J., Rice, R. W., Ann. Rev. Mater. Sci., 14, 297-334 (1984).
Boron nitride (BN) is an important ceramic material because it has the same structure and bonding as graphite. Due to this similarity, BN has stiffness, strength and ablation characteristics that are comparable to those of graphite. On the other hand, BN, an excellent dielectric material, has a very low dielectric constant and a low dielectric-loss factor in contrast to the high values of these two properties of graphite. BN also has a greater resistance to oxidation than graphite. Thus, BN provides the potential of making an oxidation-resistant body with strengths, stiffnesses, and ablation resistances comparable to those found in graphite.
Boron carbide (B.sub.4 C) is an equally important and useful ceramic material. Due to its thermal and electrical conductivity properties, boron carbide finds utility as a high temperature thermoelectric material. Wood, C., "Boron Carbides as High Temperature Thermoelectric Materials," presented at AIP Conference Proceedings 140, Albuquerque, NM, 1985. Boron carbide also finds use in applications including abrasion resistors, refractory materials and as a reinforcing agent in composites.
There have been attempts to utilize polymer pyrolyses as low temperature alternatives to the preparation of BN and B.sub.4 C. For example, mixed B.sub.4 C--SiC ceramics have been obtained from pyrolysis of poly(carboranesiloxane) polymers (Walker, B. E., Jr., Rice, R. W., Becher, P. F., Bender, B. A., Coblenz, W. S., Ceram. Bull. 62, 916-923 (1983)); however, the inventors are unaware of any disclosures relating to synthesis of pure B.sub.4 C from polymeric precursors. Ceramic materials rich in boron and nitrogen have been obtained from pyrolysis of appropriate precursors. Inert atmosphere pyrolyses of various borazine derivatives, for example, led to reasonable yields of ceramic materials containing boron and nitrogen, but the materials also contained substantial amounts of carbon. Bender, B. A., Rice, R.w., Spann, J. R., Ceram. Eng. Sci. Proc., 6, 1171-1183 (1985). Oligomerization reactions of substituted borazines with silylamine crosslinking groups have been found to provide useful gel materials which upon pyrolysis form boron nitrogen materials. Narula, C. K., Paine, R. T., and Schaeffer, R., Mat. Res. Soc. Symp. Proc., 73, 383-388 (1986), and Narula, C. K., Scheaffer, R., Paine, R. T., Datye, A., Hammetter, W. F., J. Am. Chem. Soc., 109, 5556-5557 (1987). It has recently been reported that ammonia pyrolysis of decaborane polymers linked by diamine molecules produced crystalline BN of high analytical purity. (Presentation of Rees, W. S., Jr. and Seyferth, D. at the 194th National Meeting of the American Chemical Society, September 1987, Paper INOR 446.) Also, the synthesis of BN from ammonia pyrolysis of soluble polyborazine compounds has been disclosed by Paciorek, K. J. L., Harris, D. H., Schmidt-Krone, W., and Kratzer, R. H., Technical Report No. 4, 1987, Ultrasystems Defense and Space Inc., Irvine, CA.
Despite these known methods for preparing B.sub.4 C and BN ceramics by pyrolysis of appropriate precursors, there remains a need for new precursors and improved methods which will provide the ceramics in greater purity and higher yields than heretofore possible and which will offer other advantages over prior art processes. Processible ceramic precursors are greatly desired because they would allow the ceramic to be used in a variety of applications not presently commercially feasible. For example, if a soluble precursor to BN or B.sub.4 C were available, thin films of the solubilized precursor could be cast and pyrolyzed to yield thin films of the ceramic material. Similarly, a variety of substrates could be coated with the soluble precursor material by various dipping or spraying techniques to yield, after thermal annealing, a substrate coated with the desired BN or B.sub.4 C ceramic material. The soluble precursor might also be used to prepare spun fibers of ceramic material or in preparing a multitude of various ceramic/fiber composites.
It is an object of this invention to provide ceramic precursor polymer materials rich in boron and carbon and/or nitrogen which can be pyrolyzed under mild conditions to yield BN and/or B.sub.4 C ceramics in high purity and high yields. It is a further object of this invention to provide such ceramic precursors which are processible. These and other objects will be made clear from the following summary and discussion of this invention.